A standard switching power supply employs PWM (pulse width modulation) to transfer an input power to a loading for supplying a suitable power. Switches (in general, being power MOSFETs) controlled by the PWM convert a DC input to a series of voltage pulses, and then a transformer and a fast diode are employed to output a smooth DC output. The voltage of the DC output is immediately compared with a reference voltage (the reference voltage is a predetermined output voltage for the power supply) and the difference between the voltages is feedback to a controller of the PWM for changing the pulse width according to the voltage difference. For example: when the output voltage is too high, the pulse width will be reduced to lower the supply of the power supply for returning the output voltage to the predetermined voltage. Hence, changing the pulse width to control a turning-on time of power switch can modulate a precise DC output voltage for demand.
An insufficient switching is a main reason of a power loss of a converter. When a switch is under state of turning-on and turning-off, a power consumption is occurred if a voltage and a current passing the switch are not zero. When the switching frequency of the switch increases, the average power consumption will increase because of too many transferences. A higher switching frequency can reduce the dimensions of filters and transformers and then the converter will become smaller and lighter. In a resonant converter, switching actions thereof are occurred when a voltage and/or a current is zero for avoiding the voltage and the current simultaneously being under the transferring state and so the switching losses can be avoided.
A controlling method of soft switching pulse width modulation combines the advantages of the resonant converter and the pulse width modulation. Therefore, the soft switching of a power switch and a high efficient working of high frequency can be achieved simultaneously. Furthermore, a dimension of a passive device can be reduced and a power density can be raised. This is one topic of recent development in Electrical and Electronic Systems Engineering. Researches on a soft switching of a phase shifted full bridge converter are popular in the filed of DC/DC converter, and it's a ideal topology for a high frequency DC power supply, especially on applications of medium and large power.
FIG. 1 illustrates a typical phase shifted full bridge converter. L1 is the resonant inductance outside of the transformer. An output parasitic capacitor of the MOSFETs Q3 and Q4 of the lagging bridge leg is charged with energy saved in resonant inductance L1 for reaching the ZVS (zero voltage switch) of the MOSFET. Meanwhile, due to resonant inductance L1, a reflected loading current and a reflected reverse recovery current in the primary side of the transformer pass through resonant inductance L1 and limit a rate of changing current di/dt of output diodes D3 and D4 when current transition. Hence, the reverse recovery current of the diode and the EMI (electromagnetic interference) on the circuit can be reduced.
Nevertheless, some negative influences are made by the resonant inductance outside the transformer. Generally, the quantities of the resonant inductance L1 outside the transformer are higher than the leakage inductance of an isolated transformer T for enlarging the range of the soft switching. Therefore, in FIG. 1, if the clamp circuit RCD (the resistance Rc, the capacitor Cc and the diode D1 are in the region surrounded by the dash line) does not installed on the secondary side of the transformer T, the reverse recovery current in response to a reverse recovery current of the output diode D3 or D4 flows through the resonant inductance L1. Therefore, the resonant inductance L1 stores most of energy of the reverse recovery current. Because the diode with the reverse recovery current is cut off suddenly while the reverse current reaches its maximum, an oscillation is occurred between the resonant inductance L1 and the parasitic capacitor of this diode. The voltage oscillation at point C shown in FIG. 1 is happened. FIG. 2 shows the measured voltage oscillation. Due to the voltage oscillation at point C being reflected to the secondary side of the isolated transformer, the voltage oscillation is also occurred on the diode D3 or D4 under reverse recovery as shown in FIG. 3.
A loss clamp circuit is often used to reduce the voltage oscillation. A typical loss clamp circuit RCD is presented in the region surrounded by the dash line in FIG. 1. The voltage oscillation between the resonant inductance L1 and the parasitic capacitor of diode D3 or D4 can be reduced by installing the clamp circuit RCD in the secondary side of the transformer T shown in FIG. 1. The voltage waveform of diode D3 or D4 in the secondary side with the clamp circuit RCD is shown in FIG. 4. Compared with FIG. 3, peak voltage of the diode D3 or D4 is lowered, but some parasitic oscillations still exist. Hence, the effect of the voltage clamp circuit using loss clamp circuit is not perfect.
In U.S. Pat. No. 5,198,969, the phase shifted full bridge converter with a primary side clamp circuit is provided by Redl et al. in 1992, as shown in FIG. 5. Though the mentioned voltage oscillation can be reduced by the primary side clamp circuit, some heating problems of the clamp circuit diode D1 and D2 are found because of the higher forward current flowed through clamp diode D1 and D2. Thus, problems of heat diffusing on clamp diode D1 and D2 need to be solved. Moreover, high power loss exists due to the high reverse recovery current.